diff --git a/app/models/case_log.rb b/app/models/case_log.rb
index bfcf09913..8393a5c8e 100644
--- a/app/models/case_log.rb
+++ b/app/models/case_log.rb
@@ -249,7 +249,13 @@ private
     self.totchild = get_totchild
     self.totelder = get_totelder
     self.totadult = get_totadult
-    self.tcharge = brent.to_f + scharge.to_f + pscharge.to_f + supcharg.to_f
+    if %i[brent scharge pscharge supcharg].any? { |f| public_send(f).present? }
+      self.brent ||= 0
+      self.scharge ||= 0
+      self.pscharge ||= 0
+      self.supcharg ||= 0
+      self.tcharge = brent.to_f + scharge.to_f + pscharge.to_f + supcharg.to_f
+    end
     self.has_benefits = get_has_benefits
     self.nocharge = household_charge == "Yes" ? "No" : "Yes"
     self.layear = "Less than 1 year" if renewal == "Yes"
diff --git a/spec/models/case_log_spec.rb b/spec/models/case_log_spec.rb
index e3f4d784b..917245929 100644
--- a/spec/models/case_log_spec.rb
+++ b/spec/models/case_log_spec.rb
@@ -956,7 +956,6 @@ RSpec.describe CaseLog do
   end
 
   describe "derived variables" do
-    require "date"
     let(:organisation) { FactoryBot.create(:organisation, provider_type: "PRP") }
     let!(:case_log) do
       described_class.create({
@@ -1029,6 +1028,18 @@ RSpec.describe CaseLog do
       expect(record_from_db["year"]).to eq(2021)
     end
 
+    context "when any charge field is set" do
+      before do
+        case_log.update!(pscharge: 10)
+      end
+
+      it "derives that any blank ones are 0" do
+        record_from_db = ActiveRecord::Base.connection.execute("select supcharg, scharge from case_logs where id=#{case_log.id}").to_a[0]
+        expect(record_from_db["supcharg"].to_f).to eq(0.0)
+        expect(record_from_db["scharge"].to_f).to eq(0.0)
+      end
+    end
+
     context "when saving addresses" do
       before do
         stub_request(:get, /api.postcodes.io/)